This invention relates to an EGR (Exhaust Gas Recirculation) control system for controllably recirculate a portion of exhaust gases of an internal combustion engine back to the combustion chamber of the engine.
It is well known in the art that a part of the exhaust gases of an internal combustion engine is recirculated back to the combustion chamber of the engine in order to suppress the maximum temperature of the combustion taken place in the combustion chamber to reduce the emission level of nitrogen oxides (NOx) which are generated during the combustion in the combustion chamber. By virtue of this exhaust gas recirculation, the NOx emission level has thus effectively been lowered. However, the recirculated exhaust gas considerably affects the combustion in the combustion chamber and the driveability of engine and therefore its amount is desired to be strictly controlled in response to engine operating conditions.
In this regard, the following EGR control system has been proposed by the same applicant as the present application: An EGR control valve is closeably disposed in an EGR passageway connecting an intake passageway and an exhaust passageway of an internal combustion engine. The EGR control valve is operated to control the exhaust gas recirculated back to the combustion chamber of the engine, which is accomplished by varying the exhaust gas pressure in the EGR passageway upstream of the EGR control valve in accordance with the variations of a venturi vacuum in the intake passageway. By this EGR control system, the recirculated exhaust gas flow is prevented from being affected by the variation of exhaust gas pressure in the EGR passageway. As a result, the recirculated exhaust gas flow can be controlled only in accordance with the venturi vacuum which is highly reliable as a function of the flow amount of the intake air conducted through the intake passageway into the combustion chamber. This EGR control system makes it possible to effectively decrease NOx emission level without causing the degradation of the engine driveability.
However, the above-mentioned EGR control system has encountered the following problems when used with engines which do not have a carburetor having a venturi, such as engines equipped with an electronically controlled fuel injection system in place of a carburetor: since it is difficult to obtain a highly reliable vacuum signal which acts to vary the exhaust gas pressure in the EGR passageway upstream of the EGR control valve, an accurate EGR control has failed, degrading engine driveability, fuel consumption and the ability of exhaust gas purifying devices. Of course, it may be considered to provide a venturi in the intake passageway of such a type of engines and to take out the vacuum signal from the venturi. However, with this arrangement, the flow resistance of intake air through the intake passageway is unavoidably increased to decrease engine power output particularly under high load and high speed engine operating conditions.